To add a CAPTCHA challenge to a Webform, do the following:

While in Edit mode for your webform of choice, look up in the address bar to determine the node id number.  It will be the number that comes after the node/. Jot that number down.

Go to Admin menu > Configuration > People > CAPTCHA.

Enter the Form ID into the blank field in the Form ID column. For webforms, you will want to use the form ID webform_client_form_nid, where nid is the ID number that you located earlier.

Select reCAPTCHA from the challenge type column. (Please note, the Math challenge, on the surface, often appears to be friendlier to your users, but it's not as effective as the reCAPTCHA - bots can often beat this challenge).

Click Save Configuration when you're finished.

When completed, to test your new CAPTCHA, you'll need to either go into Private/Incognito mode or use a completely separate browser that you're not logged in on.  Your result will look similar to below.